1. Field of the Invention
The present invention relates to long distance quantum communication.
2. Brief Description of the Related Art
Quantum communication holds promise for transmitting secure messages via quantum cryptography, and for distributing quantum information. See N. Gisin, G. Riborty, W. Tittel, and H. Zbinden, Rev. Mod. Phys 74, 145 (2002). However, attenuation in optical fibers fundamentally limits the range of direct quantum communication techniques, and extending them to long distances remains a conceptual and technological challenge. See G. Brassard, N. Lutkenhaus, T. Mor, and B. C. Sanders, Phys. Rev. Lett. 85, 1330 (2000). In principle, photon losses due, e.g., to attenuation, can be overcome by introducing intermediate quantum nodes and utilizing a so-called quantum repeater protocol. See H. J. Briegel, W. Dur, J. I. Cirac, and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998). A repeater creates entanglement over long distances by building a backbone of entangled pairs between closely-spaced nodes. Performing an entanglement swap at each intermediate node (see M. Zukowski et al., Phys. Rev. Lett. 71, 4287 (1993)) leaves the outer two nodes entangled, and this long-distance entanglement can be used to teleport quantum information (see C. H. Bennett et al, Phys. Rev. Lett. 70, 1895 (1993) and D. Bouwmeester et al., Nature 390, 575 (1997)) or transmit secret messages via quantum key distribution (see A. Ekert, Phys. Rev. Lett. 67, 661 (1991)). Even though quantum operations are subject to errors, by incorporating entanglement purification (see C. Bennett et al., Phys. Rev. Lett. 76, 722 (1996) and D. Deutsch et al., Phys. Rev. Lett. 77, 2818 (1996)) at each step, one can extend entanglement generation to arbitrary distances without loss of fidelity in a time that scales polynomially with distance. For comparison, direct communication scales exponentially, making it impractical for long distances. While approaches to quantum repeaters based on many quantum bits (qubits) at each node (see B. B. Blinov et al., Nature 428, 153 (2004) and S. J. van Enk, J. I. Cirac, and P. Zoller, Science 279, 205 (1998)) or on photon storage in atomic ensembles (see L. M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature 414, 413 (2001)) are now being explored, realization of a robust, practical, system that can tolerate all expected errors remains a difficult task.